Secret communication



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is: W n 52 con p 0 mu 0.0 1 $3 m Q a N m 2 82 902 F u Y Q L e m mp-11' //7 ven for Joseph W HoF/on. by Af/j/ Patented Apr. 12, 1927.

TED STATES; PATENT OFFICE.

JOSEPH W. HORTON, OF BLOOMFIELD, NEW JERSEY, ASSIGNOR TO WESTERN ELEC- TRIO COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

SECRET COMMUNICATION.

Application n ea July 2, 1923. Serial No. 848,808.

I The present invention relates to method and means for the transmission of messages with secrecy.

The invention is based on the principle that the intelligibility of received Waves depends upon the reception of a certa n ser es of frequency components occurr1ng 1n qu te a definite order between definite limits, with certain relative amplitudes, and to the practical exclusion of other frequency components. The degree of intelligibility may be reduced by disturbing the order of frequency of the components, or by changing the frequency limits or by deletingsome of the frequency components or varying their amphtudes or by introducmg extraneous or noise components. If the noise components are within the range of essential signaling frequencies so as to be incapable Of separation from .these frequencies by filtering and if they are of large amplitude, or if a material displacement in absolute frequency occurs in the components that are received, even though no essential signal component-s be deleted, the received waves may readily be made impossible of understanding as received, and also extremely d flicult to decipher b any apparatus, in the absence of knowle ge of the scheme by which the wave is built up.

The invention is particularly apphcable to systems in which the signal is transmitted as a modification of a high frequency mandible wave which must be detected to enable the 86 signal to be heard. In the detecting process a number of frequency components are produced by combination of frequency components present in the high frequency wave. By properly choosing and introducing con- 40 fusion'components into the high frequency wave, it is possible to predetermme the audible components resulting from detection and to make them of such frequency and amplitude that it is a matter of great diffieulty, amounting. to practical imposslbility, for an unauthorized person to unravel the detected and unintelligible audible wave.

The process of detection of a signal-modified high frequency wave to give the signal depends upon the interaction between the variation components of the high frequency wave, which variations arise from the control of the carrier wave by the signal, and

' the non-variation or unmoi ulated high frequency wave component. In the usual signal .modifiedwave the relation between the components is such that upon detection of the wave, components occupying the normal signal frequency range are produced. This is so by the very nature of the case, since the components existing in a signal-modified wave are themselves determined by the process of modulatou of the unmodulated high frequency wave by the signal so that the detecting process may be looked upon as a reversal of this modulating process, where-. by the reproduced signal is in exact correspondence with the original signal.

The invention takes advantage of the necessity for a proper frequency relation to exist between components of a received wave if the detected wave is to correspond with the original signal, and one manner of taking'advantagc of this is by introducing into the modulated wave such components as will prevent correspondence between the detected components and the original signal.

One of the objects of the invention, therefore, is so to alter the transmitted si nalmodified wave that the components resu ting from detection lack correspondence with the si nal, to an extent amounting to unintelligi ility. v

An auxiliary object is to introduce confus1on or noise components into the transmitted wave, so that, apart from or together with the lack of correspondence between the original signal and the reproduced components, additional components of large amplitude are produced, which tend to confuse and drown out any otherwise understandable components.

According to one feature .of the invention a single frequency component, different from the high frequency component or carrier that 95 is'modulated, is introduced into the trans- I mitted wave. This component may be termed a false carrier since-upon detection it causes the production'of components I shifted in frequency from the frequencies occupied by the components normally produced by interaction with the true carrier.

In accordance with another feature of the invention the true carrier component of the transmitted wave is suppressed and one or more false carriers are introduced into the transmitted wave.

According to another feature, the false carrier or false carriers is or are shifted in frequency from time totime to render the discovery of the false carrier frequency more difiicult.

Other features of the invention comprise method and means by which the introduction.

of the false carrier or, in general, of the confusion components, is controlled at a transmittingpoint, and the effects of such components in eflacing or masking the intelligibility of the received waves is neutralized at the point at which the signal is anthorized to be received in intelligible form.

Briefly and specificall stated, the invention involves the intro uction at a transmitting point of one or more false carriers or confusion components, and at a receiving point the neutralization of the deleterious effects of such introduced components upon the intelligibility of the received components. Thls neutralization may be carried out under control of the false carriers themselves. The received wave is detected and one of the com onents resulting by de tection from the in se carrier component is used as a base frequency from which may be built up components identical with the true carrier and the one or more false carriers, respectively. The components so built up are used to reproduce the signal and to neutralize the effects in the reproduced signal of the received false carrier or confusion components.

The various objects and features of the invention and its exact nature and operation will be made clear from the following detailed description in connection with the accompanying drawing.

In the drawing, Figs. 1 to 6 show diagrams illustrative of the frequency relations involved in the system of the invention; Figs. 7 and 8 are schematic circuit representatlons, respectively, of a transmitting sta tion and a receiving station according to the invention; and Figs. 9 and 10 show alternative transmitting and receiving circuits, respectively.

The usual carrier or radio-frequency wave modulated by speech, noise, or other sound waves, is known to comprise in general a frequency corresponding to the unmodulated frequency of-the carrier wave itself and two side bands of frequencies each equal in width to the actual width of the band of frequencies making up the sound waves, one of these side bands extending into the frequency range higher than the carrier frequency and the other side band extending into the range lower than the carrier frequency. Thus in Fig. 1 0 represents the carrier frequency, and the horizontal arrows represent the extent either sideof the carrier frequency to which the side bands reach. The side bands are indicated as being of width s,-this refer ring to the frequency range utilized in speech transmission. .The absolute frequencies comprised in aspeech modulated wave may, therefore, be designated 0, 0+8, and 68. When such a wave is detected the sum frequencies 20, 20+s, 20-8, and also double frequencies result as the higher components due to the square law action of the detector, these frequencies being ordinarily above audibility, and the difference-frequencies a and 28 result as the low or audible-fret uency components fromthe same action of the detector. Many other frequency componentsmay result but are of no practical concern in the usual type of signaling system. The frequencies 28 are ordinarily of very small amplitude relative to the frequencies 8, so that for practical purposes it may be considered that only the band 8 is received.

According to the invention one or morefrequency components is or are introduced into the modu ated wave in such a way as to render it incapable of being received and understood by the types of receiving circuit generally known or used. In Fig. 1 for example, the unmodulated carrier component a of the wave to be transmitted is preferably suppressed, this being indicated by the these introduced frequencies or false carriers are used, they are preferably positioned unsymmetrically with respect to the carrier frequency and may lie in the two speech side bands or outside the frequency limits of these side bands. -The component situated at the interval f for example, is shown nearer to the frequency of the unmodulated', carrier 0 than is the component situated at the interval f;;. The frequency components of such a wave may, therefore, be represented as-c+s, 0-8, c+f and c-f the carrier component a being assumed, as stated, to have been suppressed.

The principal frequency components resulting from the simplecor non-heterbdyne detection of such a wave are diagrammatically shown in Fig. 2. The sum-frequencies are all higher than the carrier so that these components as well as the double frequency components will ordinarily be inaudible and need not be considered. The difference-frequency components are seen to be:

band. These'latter components might at first sight be thought to represent the double frequencies of speech, but while the double frequency component of each speech compospeech side bands by themselves, may there-.

fore be disregarded. It is furthermore to .be noted that these components are of very low amplitude compared with the components produced as a result of introducin the false carriers as will presently be described, and that these stronger components completely drown out the weak double frequency components of speech.

The components may be interpreted as representing, respectively, four side bands based on f and f each of which (except for any doubling back of a band upon itself or overlapping of one portion upon another, as ex-plained hereinafter) may be considered to be of the width 8 or of theyyidth' of the n ormal'speech band, and the single component representing the sum of the two frequencies and f In the diagram of Fig. 2 these components are represented at convenient distances above the horizontal axis, their numerical frequency values being determined from their position in the horizontal direction. tempt is here made to indicate their relative amplitudes. Two side bands of speech width extend in the positive direction from the respective components 7",, and f these bands being f s and f3+8 respectively.

The side band f s and f 8, may be interpreted as being, in part, simply speech bands displaced in the downward direction.

in the frequency scale by the amount A or i as the case may be, and as being, 1n part,

inverted in their frequency orders in that portion of the normal speech range extending from zero to or f,;, respectively, the inverted portion being, therefore, superposed on the displaced portion. These partly inverted and partly displaced side bands may therefore be considered as doubled back upon themselves. The component f +f represents a single-pitched tone and for the frequencies of f and f assumed, this tone lies within the normal speech frequency limits.

No one of these resulting audio-frequency components is capable of producing understandable speech. The double-frequencies come, perhaps, the nearest to being capable of being understood but, as pointed out above, these are intermixedwith a large number of noise components'and are of such small amplitude that they are drowned out by the other components. ing normal speech as a whole upward or downward in the frequency spectrum by I The components re- It is known that shifteven a very slight constant amount introduces very serious distortion and a material frequency shift renders it'incapable of being understood. However, it will be noted that in addition to the incapability of any single one of these bands to reproduce speech, the frequencies may easily be sociosen (as is indicated) that each band overlaps the others in the occupied frequency range so thata still greater confusion resalts from the practical impossibility of dis 7 tinguishing one band from another either by ear or by any usual apparatus such'as filters.

If it be attempted to receive the waves of the character shown in Fig. 1 by the heterodyne method or by the homodyne method, sometimes referred to as zero beat reception because the locally introduced oscillations are of the same frequency as the carrier component, the same kind of difficulties are encountered as described in the case of simple detection. The use of a local wave materially difl'erent infrequency from the received frequencies will result in even more.

confused components than are indicated in Fig. 2 since the audiblcbeats produced between the'heterodyne wave and the received components simply serve to add more of the components of equally meaningless character. It is improbable that one would 10- cate the exact frequency of the unmodulated carrier and adjust the local oscillator to that the speech by any ordinary methods. If

the amplitude of the locally supplied wave is increased in an attempt to make the speech louder, this results in making the noise components louder, and moreover, the false carrier components 'may purposely be made of much higher amplitude than the speech side band components in the wave that is sent out. so that a complete masking of the speech results. lihood of locating the exact carrier frequency is lessened by the fact that A and f are located unsymmetrically with respectt-hereto. An observer in exploring the frequenciesof the wave would not be inclined to choose a beating frequency which gave The likeaudible tones in place of one corres onding to a beating frequency half way tween c-l-f and c- -f and such abeating frequency would give eifectssomewhat similar to those of. Fig. 2 where two of the side hands are partly inverted. If the local oscillator communicating by radio waves, but it willbe obvious that they might equally well be associated with a transmission line of any desired type.

The oscillator OS may be of any suitable type and is indicated as comprising an electric discharge device provided with afeed back coupling and a frequency-determin ng circuit in accordance with usual practice.

An oscillator suitable for this purpose is dis- 1 closed in United States. patent to Hartley No. 1,356,763, October 26, 1920. The harmonic generator HG may be of the type described in the U. S. patent to Kendall No. 1,446,752, February 27, 1923. As specifically shown in the present system, it is provided with a high resistance in the grid circuit for promoting the production of harmonics. The wave from the source OS is impressed on the circuit HG with such am litude as to produce a distorted wave in t e output circuit of the harmonic generator, the wave shape being such that the wave is rich in harn'lonics. The other harmonic generators indicated throughout the system, such as HG HG,, etc., may each be similar in construction and operation to HG and to the disclosure of the Kendall patent.

Various filters F,, F,, etc., are indicated at various parts of the system for selectively transmitting a wave of a. particular frequency or range of frequencies and for sup:

, pressing transmission of waves of other frecomponent.

quencies. Each ofthese filters may be constructed and proportioned in. accordance with the disclosure in U. S. patent to Campbell No. 1,227,113, May 22, 1917.

Balanced modulators BM BM etc., are

used for )roducing modulation of one Wave in accor ance with another, and for suppressing transmission of the unmodulated These modulators may be of the general type disclosed in U. S. patent to Carson No. 1,343,306, June 15, 1920, orin U. S. patent tov Hartley-No. 1,419,562, June 13,1922. Y

T-he detectors or demodulators D D (Fig.8) are of the usual three-element discharge type, and the modulator M is of similar construction, the purpose of both these detectors and this modulator being to produce sum and difl'erence'frequency com-- ponents of the impressed. wave-components. A modulatin circuit of this type is disclosed in U.; patent to Van der -Bijl No. 1,350,752, August 24, 1920.

The description from this point on, will be simplified by reference to particular frequency values, and these values are indicated on the drawing. It is to understood,

frequency which for conveniencewill be assumed to be 100 cycles per second, and this wave is impressed on theharmonic generator I-IG with such amplitude as to overload the harmonic generating tube and produce a highly distorted wave in its out put. This harmonic generator operates, in the manner disclosed in the Kendall patent referred to, to produce a number of harmonic frequencies of the impressed wave, and the construction and operation may be such that all of the harmonics from the double frequency of the impressed wave up to harmonics higher than the 10th harmonic are produced. These harmonics may be filtered out from the output circuit of the harmonic generator for separate use as maybe desired. The filter F which may be a band liltcr having a narrow transmission range or may be simply a sharply tuned circuit, is designed to select the frequency 1000 and impress it upon the harmonic generator HG suflicient' amplification being intro duced ifnecessary so that the harmonic generator HG, is overloaded and produces a highly distorted wave in its output. The

filter F selects the harmonic frequency amplitude proportional to that of the control waves and the resulting modulated wave is resolvable into two side bands having fre- .qluencies higher and lower respectively than 1e impressed 10,000 cycle wave but containing no component ofthe 10,000-c cle frequency. This action is more fully escribed in the Carson patent above referred to.

- The filter F, selects from the output of the harmonic generator HG the wave of frequency 600, which is applied to a con-- trol winding of the balanced modulator (ill the respective modulators RM, and BM...

The effect of the waves applied to the modulator BM, is to produce two side bands of the 10,000 cycle wave. these side hands being made up ,of frequencies corresponding to the impressed speech and each containing a single, frequency component spaced in freequencv 600 cycles from the center of the wave, which would be occupied'by' the 10,000 I cycle component if this were not suppressed. Similarly the modulator BM produces two side hands of the 10.000 cycle waves, these side bands re )resenting speech componen s with a single irequency introduced into each side hand at an interval of 1100 cyc'les from the 10,000 cycle frequency. Suitable ampliiication may be introduced with the filters F and F so that the single frequency components corresponding to the 600 cycle and 1100 cyclepoint in the side bands respectively are of relatively large amplitude compared with the speech side band amplitudes.

The character of the wave produced by the modulator BM, is indicated in Fig. 3.

Here the 10,000 cycle component is indicated by a dotted line signifying that this compo- .nent is suppressed. The modulated wave is made up of the two side bands indicated as extending approximately from the 10,000 cycle position to the 12,500 cycle and the 7.500 cycle limits respectively. At intervals 000 cycles from the 10,000 cycle point in each direction the respective single frequency components of relatively large amplitude are located. Fig. 4 in similar manner indicates the composition of the wave in the output of the modulator'BM this Wave (littering from that produced by the modulator BM in the different position occupied by the single frequency component or false carrier.

The filter F is proportioned in the manner indicated in the Campbell patent, re-- ferred to. to selectively transmit the lower,

side band produced by the modulator BM but to suppress transmission of the upper side band, that is, this filter transmits only that portion of the wave indicated in Fig. 3 which lies to the left of the 10,000 cycle frequency. Conversely. the filter F transmits only the upper side band of the waveproduced by the balanced modulator BM that is, only that portion of. the wave in Fig. twhich lies to the right of the 10,000 cycle line. The waves. selectively transmitted by these filters are impressed on the control circuits of the radio transmitting modulator BM,.' The character of the resultant wave supplied from these'two filters to the modulator BM is indicated in Fig. 5 and is seen to consist of two speech side hands of the 10,000 cycle wave, the upper side band containing a noise component. situated 1100 cycles higher than the 10,000

frequency and the lower side band contain-- ing a nolse component situated 600 cycles v lower than the 10,000 frequency. The neutral branch of this modulator is supplied .with a 100,000 cycle wave produced in the harmonic generator HQ, and selected by the filter F Suitable amplification may be introduced into any. of the waves supplied to theunodulator BM, as may be' desired.

The modulator BM operates in a manner similar to that described of-the otherv ba-lanced modulators to produce two side bands resulting from modulation of the 100,000 cycle wave by the waves from the filters F and F The character of the lower side band resulting from this modulation is indicated in Fig. 6. This wave is seen to have i the same frequency arrangement as the wave indicated in Fig. 5, except that the frequency order of the components isinverted and the inverted wave, as a whole, is shifted in the absolute frequency range to such position that the center of the wave is at 90,000 cycles rather than at 10,000 cycles. An upper side band with its center at the frequency 110,000 18 also produce? but the transmitting antenna 6 is prefe as to radiate only the lower side band and to suppress practically completely the upper side ban It will be noted that the composition of the wave in Fig. 6 which is the wave actually transmitted from the antenna 6, is of the same character as that indicated in Fig. 1, where C is now 90,000 cycles, A is 600 cycles and f is 1100 cycles. If this wave as transmitted were detected by the ordinary methods, either simple detection or detection in this case 1700 cycles. This component is selected by the filter F and after suitable amplification, if desired, is impressed on the modulator M, which, together with the filter F, the harmonic generator HG, and the filter F forms a sub-multi le frequency generator of the type describe in my appli- 1922. The principle of operation of this cir-. cuit may be described briefly as that of a generator in which the feed-back from the ably quite sharplytuned so cation Serial No. 545,562, filed March 21,.

with the incoming frequency of 1700 to pro; duce the difference frequency of 100, this being in turn supplied to the harmonicgenerator HGr and causing the action to continue. f

The result of the action of thesirb-multiple frequency generating circuit causes the harmonic generator I-IGr to produce a number of components of the impressed 100 cycle wave under control of the received 1700 frequency wave. The filter F selects the 1000 requency Wave and applies it to the har- ,IIlOlllG generator HG from the output of which the filter F selects the 10,000 cycle wave which is supplied to the neutral branches of the respective balanced modulators EM, and BM The filter F selects the 600 cycle component from the harmonic generator HGr andsupplies it to the control windings of modulator BM and the filter F selects the 1100 cycle component and similarly supplies it to the control windings of modulator BM 1 The action of the modulator BM, is. to produce two side frequencies of 600 cycles above and below the 10,000 frequency re spectively and havingtherefore the respective frequencies 10,600 and 9,400. The filter F selects for transmission only the 9,400 component, this being supplied to the circuit generally indicated by the numeral 9, the purpose of which circuit is to adjust the phase of the 9,400 cycle component in a manner to be described. The modulator IBM produces two side frequencies of 8,900 cycles and 11,100 cycles respectively, the upper one of which is selectively transmitted by the filter F to the phase controlling circuit 10.

Some of the 10,000 cycle wave from the filter F is supplied with suitable amplification to the harmonic generator HG from the output of which the filter F selects the 100,000 cycle component. This 100,000 frequency is introduced through the coupling 11 to the input circuit of the detector D which is also coupled tothe receiving antenna 7. As a result of the action of the detector D1,, the sum and difference frequency components of the 100,000 cycle wave and the received wave are produced. The

difference frequency components include a .based on a 10,000 cycle wave.

wave of the character indicated in Fig. 5. this being clear from considering that each ofthe components of the wave indicated in Fig. 6 (which is thercceived wave) is sub-. tracted from the locally supplied frequency 100,000. The,filter F is designed to pas selectively the frequencies comprised in th' ditfcrence frequency wave and this wave i thereafter impressed on the amplifying circuit A The input circuit of the amplifier A is also supplied with the 11,100 frequency componcnt from theicircuit 10 and with the 9,400 cycle component from the circuit 9. Suitable amplification or attenuation may be introduced into the several circuits associated with the input'of the amplifier A and also the phase of the locally supplied components may be so adjusted by means of the circuits 9 and 10 that the 9,400 cycle component and the 11,100 cycle component locally supplied are each of the same amplitude as the corresponding components impressed from the detector D and are each in phase opposition to these respective components. As a result, the false carriers or noise or confusion components purposely introduced into waves at the transmitter are balanced out in the arm plifier A leaving only two speech side bands These side hands, after passing through the amplifier A are supplied to the controlv winding of the'modulator BM where they interact with'the locally supplied 10,000 cycle component from the output of the filter F and produce speech currents'in receiver R in the manner. indicated .in Carsons patent above referredto.

In the foregoing description, it is stated at several points that amplification may be introduced where desired. No attempt has been made to indicate all'of the points where amplification maybe desirable but it will be understood' that amplifiers or attenuating networks, and otentiometers and the like for regulating the amplitude of the waves may be employed.

Under some circumstances, it may be desirable to render the secrecy scheme utilized still more incapable of being deciphered than is the scheme employed in the system of Figs. 7 and 8. For example, it might be desirable under some conditions, where the eneral secrecy scheme made use of might be 'nown, toemploy some means for rendering it more diflicult to discover the frequencies A .and f or false carriers. ()ne way of rendering false carrier frequencies more difficult of discovery would be to change their frequency from time to time during transmission. This method is applicable, of course, to systems in which two false carriers, such as are described in Figs. 7 and 8 are used or in systems where more than two false carriers or only one are used. If more than one false carrier is used, the circuit arrangements for providing the false carriers and shifting their frequencies becomes somewhat involved, and for the sake of simplicity and for the purposes of illustration it has been deemed sufficient to show as an example of a system with variable frequency false carriers, the system indicated in Figs. 9 and-10' in which provision is made for using at any instant a single falsecarrier and for shifting the frequency of this false carrier from time to time. However, by mere extension of the'princinle illustrated in Figs. 9 and 10, to be described, it will be clear that the frequencies of a plurality of false carriers may likewise be shifted fromtime to time, in cases where a plurality of false carriers are used.

The frequency of the false carriers could obviously be shifted in accordance with any desired scheme. For example, the frequency might be shifted in accordance with a prearranged time schedule, the corresponding shifts being efi'ected. simultaneously atv the transmitter and receiver with preferably some synchronizing system for controlling the shifting means at the two stations.

In the system of Figs. 9 and 10, however,

the shifts in frequency of the false carrier waves are effect ve under control of the speech to be transmitted, the specific arrangement being such that during a pause of sutlicient length between words a frequency shift is made simultaneously at both stations.

For the purpose of shifting the frequen- 'cies, step-by-step switches of the type coin- 1 'monly used in the automatic telephone art are indicated as being employed. In Fig. 9, for example, the rotatable switch arms 12 .may be mounted upon a single shaft ar- "ranged foractuation under-control of the stepping relay 14. Similarly, the brush arms 15 may be stepped by the relay 17 In Fig. 10, a number of pairs of brush arms 18'to- 24 inclusive..are indicated together Q-in each case by the arrow.

with a single stepping relay 25 for control- It will be obvious mounted on a single shaft or upon different shafts with suitable driving connections so that when relay 25 actuates its armature, all

of these brushes are advanced from the contact upon which they are resting to the next succeeding contact in the direction indicated The stepping relays 14, 17 and 25 are each arranged in the manner common in the automatic telephone art to effect no movement in the brushes during the upward or'attrac tive stroke of their armatures but to advance -the brushes one step during the backward stroke of the armatures. This may conveniently be arranged, for example. by havingthe armatures normally retracted by a spring and providing a pawl which upon the attracting stroke passes from engagement with one tooth of a ratchet wheel to engagement with a succeeding tooth and upon retraction of the armature moves the ratchet wheel one step.

The relays 14, 17 and 25 are normally deenergized except when speech is beingtransmitted. Speech currents directed against the microphone '26, for example,-are repeated through amplifier 27 and set u corresponding speech currents in the winm ing 28. The

grid of the. rectifier tube 29 is normally at such a high negative value that the space. current in the tube 29 is too low to cause energization of the relays l-l and 17, but the spcechcurrents in the winding 28 producing corresponding voltage changes on the grid of the tube 29 and cause an increase in the space current in this tube to such a value as causes the relays 14 and 17 to attract their armatures. At the receiving station ofFig. 10, received speech currents are impressed on the rectifier 36 which acts in a manner simithe uppermost of the three stationary contacts respectively associated with it. v

The oscillator OS in Fig. 9 and the harmonic generator HGr operate as described in connection with Fig. 7 to produce a' series of waves having frequencies which are harmonics of the base wave of 100 cycle frequency. The filter F selects 1000 cycle component and applies .it to the harmonic generator HG from the output of which the filter F selects the 10,000 cycle component and applies it in turn to the harmonic generator HG and from the output of HGr 'the 100,- 000 cycle component is selected by the filter F to be used asthe radioor carrier frequency. This component is applied to the tenna. 30.

Withtlie brushes 12 and 15 in position 1,

the filter F is connected between the har-' monie generator HGr. and the input side of the amplifier A. so thatthis amplifier is supplied with a 400 cycle component. As soon as speech is directed against the microphone 26. the amplified speech currents are supplied preferably through a second amplifier duced in the modulator, each of these comprising components representing speech and a 400 cycle noise component or false carrier. preferably of large amplitude compared with the speech components. These side bands are radiated from the antenna 30. \Vith the beginning of the conversation, the

relays 14 and 17 are energized inthe mannerpreviously described and are thereby put in readiness to advance the brushes 12 and as soon as a sufficiently long pause occurs 111 the talking, thereby permitting these relays to release.

The side hands transmitted from the antenna 30, if detected on an ordinary detector, would produce only the double frequencies of speech upon whic 1 would be superposed a very strong component of 800 cycles. As stat-ed above. it is practically impossible to understand the double frequencies of speech,

carriers or noise components, particularly if they are unsymmetrically spaced as described in the case of Figs. 7 and 8, the wave would be rendered still more difiicult of understanding. I I

The wave received on the antenna'i-ll of station 10 is detected-in the circuit 1) and produces a component of 800 cycles in the output. This component is passed through the filter F brusharms 18 and 19 being in position 1, and is impressed on the modulator M,. This modulator, together with filter F harmonic generator HG and the particular filter F F that is connected in the circuit at any 1nstant, forms a sub-multiple generator of the type described in connection with Fig. 8. \Vith brush arms 20 and 21 in position the application of the 800 cycle component an tem to the modulator M, causes the production m the modulator of the difierence frequency between the 800 cycle component and the 700 cvcle component transmitted through the tilter F so that a component of 100 cycles is maintained in the input circuit of the harmonic generator HG I i The filter F selects the 1000 frequency component from the output of the harmonic generator HG and applies'this component to generator HG,,. The 10,000 cycle component' occurring in theoutput of the harmonic generator HG is selectively transmitted by filter F to harmonic generator HG from the output of which the 100,000 frequency component is applied through filter F, to the input circuit of detector D selectively Thisdetector serves to combine the received wave components with the locally supplied 100,000 cycle component to produce speech frequencies with the 400 cycle noise com )0- nent superposed thereon, and the speechrequencies are selectively transmitted through thiscomponent exactly neutralizes the 400 cycle noise component in the detected speech wave, and the speech in understandable form is transmitted through the amplifier 32 and speech transformer 35 to the receiver R.

\Vhen the conversation started, the speech currentspresent in the transformer 35 were impressed on the rectifier 36 which is controlled in a manner entirely analogous to that described in connection with rectifier 29, Fig. 9, and the resulting increase in space current energized relay 25 causing its armature to engage a tooth on the driving mechanism for the various brush arms indicated in Fig. 10 and to hold the armature attracted so long as no pause of less than predetermined length occurs in the conversation. When a pause of sutficient length occurs between words or sentences, for example, the rectified current in the rectifiers 29 and 36 respectively, falls to such a low value that the relays 14, 17 and 25 simultaneously release their armatures and advance the brush arms 12, 15 and 18 to 24 inclusive, to the second position, that is. the position succeeding that in which the brush arms are shown in the drawing, in the direction of the arrows. With the brushes in the second position, the conversation when it is resumed, proceeds exactly as described above except that the false carrier or noise frequency component has been shifted to a different frequency from what it formerly had. In the example chosen for illustration,- the false carrier has a frequency 500 cycles different from the true carrier when the brushes are in position 2, that is, a 500 cycle tone is superposed on-the detected speech.

This 500 cycle tone is impressed on the outgoing speech at the station in Fig. 9 in the modulator BM and is derived by means of the filter F from the output of the harmonic generator HGr when brushes 12 and 15 are in position 2.

The frequency resulting from detection of this 500 cycle modulating component is now of 1000 cycles and this component is selected from the detectorD by the filter F brush arms 18 and 19 being in position 2. The submultiple frequency generator circuit continues to derive the base frequency of 100 cycles for supplying to harmonicgenerator HG through filier F since brush arms 20 and 21 are nowin position 2 and filter F is in circuit to feed back the 900 cycle component to combine with the incoming 1000 cycle component. ceived speech is neutralized by the 500 cycle tone selected by filter F from the output of harmonic generator HG and thephase of this neutralizing component is adjusted by means of the circuit 37, brush arms 24 being in the second position so that this neutralizing component is supplied to the amplifier 32. So long as the conversation continues without suflicient pause, relays 14, 17 and 25 remain with their driving pawls in engagement with the shafts of-the respective brush arms. A pause of sufficient length occurring in a conversation with the circuit conditions as last described above causes all of the switch arms shown in Figs. 9 and 10 to advance to the third position, whereupon the false carrier or noise component isagain shifted in frequency. In the example chosen for illustration, the new noise frequency is 600 cycles and it will be seen that brush arms 12 and 15 in position 3 c0nne ct the filter F in circuit to supply the 600 cycle component to the outgoing modulator. Also at the receiving station, filters F F F and phase adjusting circuit 38 are cut into the circuit softhat when the talking is again resumed, the neutralizing component applied to=the phase amplifier 32 is now 600 cycles and is of proper phase to neutralize the 600 cycle noise component of the received speech. When talking is resumed, relays-14, 17 and 25 are again energized in preparation for advancing the various brush arms into position 1 upon the occurrence of a sufficiently long ause. J

It will be seen that the above described arrangements for shifting the frequency of the false carriers or noise components are such that no shift in frequency is made in the middle of a Word or between words that follow each other with the normal rate of succession used in ordinary conversation. The relays 11,17 and 25 may be made sufficiently slow so that a considerable pause is necessary before a shift in the frequency of the false carrier is made, such, for example, as the pause which occurs between sentences or during a considerable break in the conversation as between the time when one party ceases talking and the distant party begins to talk.

There are two possible ways in which the frequency shifting system might get out of step, that is, first, due to failure of the The 500 cycle tone in the re-' brushes at the receiver to advance a step when the brushes at the transmltter advance, and second, due to an advance of the brushes at the receiver when no advance takes place at the transmitter. Since the stepping of the brushes at the receiver is effected upon release of the relay 25, and this relay is positively held energized by received energy, it is highly probable that the brushes at the receiver will always advance with an a'd vanceof the transmitter brushes, so that the first of the two conditions mentioned above may be considered as practically never existing. There may be interruptions in the transmission due to other causes than a pause in the conversation and lasting sufficiently long to permit relay 25 to release and advance it's brushes. If this occurs during conversation and without a simultaneous advance of the brushes at the transmitter, the receiving circuit is momentarily disabled from receiving intelligible speech. However, upon the first pause in the conversation sufficiently long to advance the brushes at the transmitteqthe two stations are again in step and conversation may proceed in the normal manner. In such a contingency the loss of a few syllables or words might occur,

but in a two-way system such as would ordinarily be employed the speaker at the transmitter could readily be asked to repeat this lost portion of the sentence. It is evident that in case of such a lack of synchronism between the mechanisms of the two stations, the missed words through lack of synchonism Would be heard as noise at the receiving sta tionso that the receiving party would know that words were being spoken and that a lapse was not occurring in the speech."

In the very unlikely event of the transm tter brush arms taking one step Without a corresponding advance of the receiver brush arms, it would be necessary for the trans-. mitter brushes to take one less than the total number of additional stepsnecessary to advance a brush from any given position through all the positions and back into that position. The receiver brush arms would, of course, be forced to wait for the transmitter brush arms to come into corresponding posi tion, since the energy necessary to move the receiver brushes is not received until the apparatus at both stations is in step.

N o provision is shown in either of the described systems for carrying on two way conversation but it will be obvious that a return channel can be provided by a mere duplication of the apparatus shown with or without the use of different frequencies at the various points throughout the system.

Provision i s'shown in Figs. 9 and 10 for only a. single false carrier or noise component but it will be obvious that a plurality of false carriers may be employed in the manner generally indicated in Figs.

' false carriers or of one or more of them may be shifted from time to time.

In the system described in Figs. 7 and 8 employing two false carriers or noise C0111- ponents, both side bands of the speech modulated wave are transmitted and one of the noise components is inserted in each side, band. It is within the invention, however, to supply a plurality of noise components to a: single side band and to transmit only this side band, suppressing transmission of both the unmodulated carrier and the opposite side band. It is also within the invention to transmit two false carriers and either a single side band or both side bands, whether the false carriers lie within the limits of the side bands or outside these limits, and to introduce frequency shifts into one or more of the false carriers.

The foregoing descriptions assume thatha'rmonics, as high as the sixteenth, are obtainable in the output of a harmonic generator but it will be obvious that the sixteenth harmonic or any desired harmonic v of a wave may be secured by means of a plurality of harmonic generators operating in tandem with proper selecting circuits between, and accordingly. in case the desired harmonic is not obtainable from a single havrmonic generator, it will be obvious to supply other harmonic generators where they may be needed throughout the system.

It is to be understood that the scope of the invention is defined in the following claims and that the invention is not to be construed as limited by the specific showing either in the drawing or the description, this being for illustrative purposes only and merely by way of example.

What is claimed is:

1. In signaling in which the signal is revealed at a receiving point by derivation from a carrier wave component and components containing signaling variations, the method of obtaining secrecy in transmission comprising replacing the carrier wave component by a component of a different frequency, transmitting the wave so altered, and deriving at a receiving point the carrier component from the component of different frequency.

2. The method of purposely reducing the degree of intelligibility of a modulated wave comprising suppressing the unmodulated carrier component of the modulated wave suppressing the unmodulated carrier component from transmission and introducing into the modulated wave a sustained wave of such frequency as to produce in the resulting modulated wave a component of the order of frequency of one of the speech side bands of the modulated wave.

4. The method of secret transmission comprising modulating a carrier wave by speech and by a confusion current of a definite frequency within the audible frequency range, and suppressing transmission of the unmodulated carrier component.

5. The method of secret transmission com.- prising producin waves of two bands of frequencies, each band of a frequency -width corresponding to essential speech frequency range and each representing the same speech, introducing in each of said bands a different character of distortion, and modulating a carrier wave in accordance with both of the distorted bands.

6. The method of secret transmission comprising modulating a carrier wave by speech waves, and introducing into the modulated wave sustained confusion components unsymmetrically related in frequency with respect to the carrier frequency and differing from the carrier frequency by an amount within the limits of speech frequencies.

7. The method of secret transmission comprising producing two bands of waves each of speech frequency width and each representing the same speech, introducing into each of said waves a confusion component of respectively different frequency, and modulating a carrier wave by said waves.

8. The method of producing a speechmodulated carrier wave containing both side bands and confusion components, which comprises modulating a relatively low-frequency wave by speech and a confusion component, selecting the upper side band of the .wave so modulated, modulating said lowfrequency wave by the same speech and a confusion component, selecting the lower side band of the wave so modulated, modulating a high-frequency wave by both of said selected side bands, and selecting for transmission one of the side bands resulting from the modulation of the high-frequency wave.

9. The method of secret transmission comprising modulating asustained wave with the same speech waves by separate modulating operations, introducing into the modulated waves so produced respectively different distortions, selecting the upper side band of one of the modulated waves and the lower sideband of the other modulated wave and transmitting said side bands togather over the same medium.

10. The method of secret telephony comprising modulating a carrier wave by speech, suppressing transmission of the un- Ill) modulated carrier component, introducing a confusion component into the modulated wave, the frequency of said confusion component lying within the frequency limits occupied by one of the speech side bands of the modulated wave, deriving at a receiving station a wave of the carrier frequency from said confusion component and utilizing said derived wave to receive speech from said modulated wave.

11. The method of secret telephony comprising modulating a carrier Wave by speech, introducing into each of the two side bands of the speech-modulated wave a confusion component, obtaining at a receiving point the beat between said confusion components, and utilizing said beat to derive components for neutralizing the confusion components in the speech to be received.

12. Means to produce a wave for secret transmission of speech comprising means for modulating a carrier wave by speech and suppressing transmission of :he unmodulated carrier frequency component, and means to superpose the modulated wave upon a sustained Wave of relatively large amplitude and of a frequency differing from the carrier frequency by an amount small enough to produce unintelligible audio waves when detected.

13. Means to produce a wave for the secret transmission of speech comprising means for modulating a carrier wave by speech and suppressing transmission of the unmodulated carrier component, means to transmit the two speech side bands of the modulated wave, means to transmit with said modulated wave a sustained wave of the same order of frequency as the frequencies of one of the side bands and a sustained wave of a frequency lying to the other side of the carrier frequency and spaced in fre-' quency from said carrier frequency by an interval different from the interval between the carrier frequency and the frequency of any sustained wave component on the same side of the carrier frequency as the first mentioned sustained wave.

14. Means to produce a wave for the secret transmission of speech comprising means to produce an upper side band of a speech modulated wave of arbitrary frequency and to distort said upper side band wave, means to produce a lower side band wave by modulation of the same arbitrary frequency wave by the same speech waves, and to distort in a different manner said lower side band wave and means to modulate a carrier wave by both of said side band waves.

15. In secret signaling, means to modulate a carrier wave by speech and means to introduce into the modulated wave a plu-- rality of confusion components of respective frequencies lying to each side of the carrier: frequency and unsymmetrically positioned in frequency Withrespect to the carrier frequency.

16. In secret telephony, means to modulate a wave of sustained frequency by speech. means to introduce into the upper side band of the wave so modulated a confusion component of a frequency within the frequency llmits of said side band, means to introduce into the lower side band a confusion component occupying a relatively different position with respect to the frequency limits of that side band, and means to transmit said two side bands containing said confusion components over a common medium to the substantial exclusion of wave components of other frequencies. I

17. In secret telephony means to modulate a wave of arbitrary frequency by speech, means to introduce into the upper side band of the resulting modulated wave a confusion component of a frequency within the frequency limits of said side band, means to introduce into the lower side band a component of a frequency within the frequency limits of that side band, raid latter component occupying a relatively different frequency location from the first component with respect to the said arbitrary frequency,

means to select said two side bands and said confusion components, and means to modulate a carrier wave thereby.

18. In a signaling system, means to mod ulate acarrier wave by speech and to suppre.;s transmission of the unmodulated car-- rier component, means to introduce a confusion component into the modulated wave, and means at a receiving point to derive a wave of the carrier frequencyfrom said confusion component.

19. In a signaling system means to modulate a carrier wave by speech and to suppress transmission of the unmodulated carrier component leaving the two speech side bands for transmission, means to introduce a confusion component of a frequency within the frequency limits occupied by one side band and a confusion component within the frequency limits occupied by the other side band, and means at a receiving point for deriving from said confusion components a wave of the carrier frequency and other components for neutralizing the effect upon received speech of said confusion components.

20. In a secret telephone system, means to modulate a carrier wave by speech waves and to introduce confusion components into the modulated wave, means at a receivin point to obtain the beat wave between said confusion components and to derive from said heat wave components for neutralizing the effect on the received speech of said confusion components. I

21. In a secret telephone system, means to modulate a carrier wave by speech and to g a carrier Wave substantially free from components of the unmodulate'd carrier frequency, means to produce false carrier waves differing in frequency from said unmodulated carrier frequency, means to transmit said false carriers and said speech modulation components over the same medium, said false carriers being so related in frequency to said modulation components as to permit of reception of only unintelligible sounds by ordinary detection, and means at a receiving point for deriving intelligible speech from said speech modulation components under control of said false carrier waves.

23. In a secret telephone system, means to produce speech side bands of a true carrier wave and to superpose saidside bands upon a false carrier Wave for transmission,-

said false carrier wave having such a frequency relative to said side bands as to permit of reception of only unintelligible sounds by mere detection of the transmitted waves, and means at a receiving point for deriving a wave of the frequency of the true carrier from said false carrierwave and for utilizing said derivedrwave to receive intelligible speech from the received waves.

'24. In asystem of secret telephony, means to produce speech side bands of a true carrier wave and to superpose said side bands upon false carrier waves for trans mission comprising a source of base frequency waves, a harmonic generator for deriving waves of intermediate frequencies from the wavesfrom said source, means for deriving higher frequency waves from one of said intermediate frequency waves, means to modulate an intermediate .fre quency wave by speech and by other intermediate frequency waves, means to select the modulation components resulting from such modulation and comprising speech side band components and components representing modulation by intermediate frequency components, means to modulate said higher frequency waves by the selected modulation components, and means to select for transmission one side band of the last resulting modulated wave.

25. In a secret telephone system, means to produce speech side bands of a carrier wave and to suppress transmission of the unmodulated carrier component, means to superpose said side bands upon a plurality of false carrier waves unsymmetrlcally poduce a wave of the frequency of the suppressed carrier and to reproduce speech currents from the side band components, substantially free from effect of said false carrier components.

26. In secret telephony, means to produce speech side bands of a carrier wave and to suppress transmission'of the unmodulated carrier component, means to superpose confusion currents upon said side bands and to transmit the side bands and confusion components together, means at a receiving point for deriving from the confusion components a Wave of the carrier frequency and waves of frequencies equal to the'frequencies of the confusion components, and means to combine the received waves with the derived waves to reproduce speech waves substantially free of the effects of said confusion components.

27. In secret telephony,'means at a transmitting point to produce a sustained wave of a base frequency and to derive therefrom a carrier wave and confusion components, means to modulate the carrier wave by speech and said confusion components and to suppress transmission of the unmodulated carrier component, means to transmit the resulting modulation components to a distance, means at a receiving point for deriving from the modulation components produced by the confusion components a wave of the base frequency, means for producing from the derived wave a base frequency, a wave of the carrier frequency and waves of the frequencies of the respective confusion components, and means to combine the waves so produced with the received waves to reproduce speech waves substantially free of the effects of said confusion components.

28. In secret telephony, means to transmita speech modulated carrier wave and confusion waves together, the confusion waves hearing such frequency relation with respect to the carrier frequency as to permit of reception of only unintelliglble sounds by ordinary detection, means at a designated receiving point for combining the confusion components to reproduce waves of the frequencies ofsaid confusion waves, and means to combine the reproduced waves with the received waves and thereby to neutralize the effect upon the intelligibility of the detected currents of said confusion waves,

29. In secret telephony, means to produce a speech modulated wave for transmission, means to superpose on said waves confusion waves, means at a receiving point to utilize said confusion waves to reproduce components for neutralizing the confusion waves to enable reception of intelligible speech, and means to change the frequency of the confusion Waves during transmission of a speech message.

30. In secret telephony, means to transmit a speech modulated wave and to superpose confusion Waves on the transmitted speech modulated waves, and means operating under control of speech currents for changing thecharacter of the confusion waves. 7

31. In secret telephony, means to transmit a speech modulated wave and to superpose confusion waves on the transmitted waves, and means controlled in response to a pause in the conversation for changing the character of the confusion waves.

32. In secret telephony, a source of high frequency waves, sources of confusion waves of respectively different character, means for interchangeably transmitting confusion waves of different character, means to modulate the high frequency waves by conversation currents and means controlled by theconversation currents for determining the particular character of confusion waves being transmitted at any instant.

33. In secret telephony, a source of carrier frequency waves at a transmitting point, means to produce a plurality of confusion waves of respectively different character, means to modulate the carrier waves by speech and by selected ones of the confusion waves, means at a receiving point to reproduce from the confusion waves, waves of the respective character to neutralize said confusion waves in their eflect upon speech reception, meansto utilize the reproduced waves to derive intelligible speech from the received waves and means at both the transmitting point and the receiving point controlled by speech currents for selecting at any instant the character of confusion waves to be transmitted and the character of neutralizing waves to be utilized respectively.

34. In secret telephony means at a transmitting point to produce a wave of base frequency and to derive therefrom a carrier wave and confusion waves for transmission, means to change the character of the confusion waves from time to time, means at a receiving point to derive from the confusion waves of onecharacter awave of said base frequency and to reproduce therefrom a wave of the frequency of said carrier wave, and means to derive from the confusion Waves of changed character at any instant a wave of the same base frequency.

35. In secret telephony, means to build up and transmit a composite wave comprising speech side bands of a modulated carrier wave and confusion waves for preventing reception of intelligible speech from the transmitted waves by ordinary detecting processes, means to change the character of the confusion currents from time to time under control of speech waves, means at a receiving point to reproduce from said composite wave Waves of the character necessary to neutralize the effect of said confusion waves upon the reception of speech to enable intelligible speech to be received, and means controlled by the received speech currents for changing the character of the neutralizing Waves in accordance With the change in character of said confusion waves at the transmitting point.

36. The method of transmission comprising producing at a transmitting point two side bands of a high frequency Wave, each of said side bands being of speech frequency width, suppressing the umnodulated component of said high frequency wave, transmitting the two side bands to a receiving point and there utilizing said side bands to produce unmodulated waves and combining said unmodulated waves with the received side bands to produce speech waves.

37. In a carrier system, the method of transmitting signals, which comprises separately modulating a carrier wave by nals derived from a single source, selecting one side band of each modulation, and

transmitting the selected side bands over a single channel.

38. A carrier system comprising means for separately modulating a carrier wave by signals derived from a single source, means for selecting one side band of one modulation, means for selecting one side band of the other modulation and means for simultaneously transmitting the selected side bands over a single channel.

In witness whereof, I hereunto subscribe my name this 22 day of June A. D., 1922-.

JOSEPH W. HORTON.

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